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From burning plasmas to liquid wall divertors

published by Merlijn on Thu, 19/12/2013 - 14:33

I couldn’t suppress a feeling of disappointment when I visited ITER for the first time a few weeks ago. Or rather: the ITER site, because there is not much to be seen yet. Even though I was aware of this beforehand, I have to admit that the little apparent progress was still a bit of a let down.

When talking about fusion with people who don’t work in the field this is a much-heard sentiment. And since they, naturally, vastly outnumber the ones who do, I often find myself in a position where I feel that I have to ‘defend’ fusion. But I can hardly blame people for being skeptical about fusion when I feel frustrated with the pace of progress myself.

The topic of my PhD

In a way, this is comparable to doing a PhD: the work can be tedious and progress slow, and the relevance is easily questioned by outsiders. Explaining what you do and why you do it can be challenging, and often I find that people are afraid of even trying to understand.

My work concerns itself with three topics, but they are all linked to the burning plasma that we expect in future fusion reactors. Current fusion reactors require constant external heating to keep the plasma at the required temperature, which you could compare to burning a peace of wood in the flame of a Bunsen burner. An energy producing reactor however, will rely largely (possibly completely) on the heat produced by the fusion reaction to keep the plasma hot, which is similar to a fire in a fireplace: once you light it with a match, it will keep itself going as long as you supply fuel and remove the ash. The same holds true for a burning fusion plasma.

Part 1. The difficulty of burning plasmas

When one wants to build a reactor to generate and control such a plasma, it is useful to know how they behave. First and foremost, the combination of density and temperature such a plasma would burn at, since the combination determines the fusion power which is of course a crucial design parameter. For lack of experience with burning plasmas, we rely on scaling laws that we use to extrapolate from present day machines to future reactors, and I concern myself with the difficulties that arise from the differences between the plasmas we’re used to and burning plasmas.

Part 2. Control of the plasma by turbulent transport

One of these differences is that the external heating is no longer an effective means to control the pressure in the plasma, as is done in present day machines, so new means of control have to be found. One possibility is to try and manipulate the turbulent transport that is responsible for the bulk of the energy loss from the plasma. From theory and experiment we know that there is an interaction between shear flows in the plasma and turbulence, and the possibility of using this to our advantage is the second part of my PhD.

Part 3. Liquid power exhausts systems

The final topic I’m working on is the exhaust of the reactor: the ash of the fusion reaction is helium and this needs to be removed. This means that in a certain area of the reactor, called the divertor, the plasma actually touches the wall. The heat load on the wall in this region is so high, that conventional metals have a hard time dealing with it. Such a hard time in fact, that with present technology we couldn’t build a reactor where the divertor would last long enough to be economically viable. A possible solution to this would be to locally protect the wall with a thin film of liquid metal, which could be continuously refreshed. The challenge with this approach lies in the fact that there is a strong interaction between the magnetic field in the reactor and flow of the liquid metal, which is something that my research aims to shed some light on.

Trying to explain all this in one minute to the average person with no background in physics, let alone in fusion, is a difficult task. Of similar difficulty I would argue, to explaining why fusion energy has been thirty years into the future for the last half-century or so. But I believe this is a vital skill to master, since achieving fusion energy is like building a cathedral; a large-scale effort spanning several generations, the reward of which might not come during your own lifetime. So in this little corner of the world wide web I will try to explain to you why fusion is worth the wait, what I’m working on to achieve fusion power faster and above all, what makes it such a fascinating challenge.

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